What about when ballots change?

Ballots are not permanent things. Every so often, the ballot itself changes, whether it's because a party changes their candidate, a new party begins to contest the seat, an old party stops contesting the seat, or an independent candidate adds or removes themselves from the ballot. When this happens, it's possible for a vote that's been cast to become invalid.

Rather than invalidate all votes when the ballot changes, forcing every voter to cast a new ballot, it's simple to determine the set of invalid ballots.

1. When a party changes candidates, there are two options. The complicated option is to invalidate any ballot that voted for the old candidate. The much simpler option is to acknowledge that party affiliation is meaningful, and count ballots with votes for the old candidate as ballots for the new one. We'll call this 'vote inheritance'.
2. When a new party begins to contest a seat, it's not necessary to invalidate any ballots at all. It's up to the new party to convince voters to change their existing votes. The same applies when a new independent candidate is added to the ballot.
   
3. When an old party stops contesting a seat, every ballot with a vote for that party's candidate must be invalidated. The same applies when an old independent candidate is removed from a ballot.

By acknowledging party affiliation and allowing vote inheritance, invalid ballots only occur when a party or independent candidate stops contesting a seat. But what do we do with invalid ballots? Simple: the vote counting agency posts the list of invalid ballot numbers to the agency in charge of the electoral roll. The electoral roll agency then correlates these ballot numbers with the voter ID and posts a notice to every voter that's had their ballot invalidated, asking them to please vote again.